Linter `parser` never called if command doesn't write to stdout before completing.

[9999years]

(Low-priority, documentation.) When writing a linter in Lua (see #816), I first attempted to set cmd = "true". true(1) produces no output and exits with code 0. I was pretty baffled that my provided parser function was never called. When I switched the cmd to echo instead, it suddenly worked! I guess the command needs to write something to the output stream for the command functions to be called.

This may be an issue with the Lua APIs somewhere (e.g. uv.read_start) rather than the nvim-lint implementation, but it's probably worth documenting.